As the degree of integration in semiconductor circuits increases, there is a concomitant need to develop semiconductor devices with high operation speed and low power consumption. To provide such devices having these excellent characteristics, it is necessary to ensure low contact resistance. As the contact dimensions of semiconductor devices shrink, however, the contact resistance typically increases, and in addition, the sheet resistivity of the shallow-junctions of the diffused regions also increases.
To reduce these resistance values, one widely used and relatively simple technique is typically referred to as self-aligned silicide (salicide). According to the salicide technique, a transition metal such as Ti, Ta or Mo is deposited over a MOS structure, and reacted with the exposed Si areas of the source and drain as well as the exposed poly-Si areas on the gate by thermal treatment, to form a silicide. During the silicide formation, oxide spacers are used to prevent the gate and source/drain regions from being electrically connected by avoiding silicide formation on these oxide spacers. After the silicide formation, a selective etch removes the unreacted transition metal without attacking the silicide, the silicon substrate or the oxide spacers. As a result, each exposed source/drain region and poly-Si gate is covered by a silicide layer.
Unfortunately, this technique has drawbacks because of a bridging effect. The bridging effect occurs because silicon from within the oxide spacers diffuses into the metal layer, e.g., Ti layer, that covers the oxide spacers and subsequently reacts with the Ti during the thermal treatment process for the silicide formation. Thus, unwanted reaction of metal with the oxide spacers results in the lateral formation of silicide which can easily bridge the separation between the gate and the source/drain regions. Also, because unreacted metal may not be totally removed by the selective etching process and may remain over the substrate additional bridging may also take place. Consequently, unwanted silicide and remaining metal can cause the gate to become shorted to the source/drain regions.